| Summary: | 碩士 === 國立交通大學 === 電子工程系 === 91 === The channel length of traditional trench gate power MOSFET is confined to punch-through breakdown of source/drain, and it is decided by the depletion width when large drain voltage is applied. Besides, the peak base concentration in channel determines the threshold voltage of the device. The p-body region of traditional process is formed by a boron diffusion from the wafer surface. Thus, the concentration near source side is high and the concentration near drain side is low. If lowering the p-body concentration near source side to reduce the threshold voltage, it will make the p-body concentration near drain side even lower. At the specification for a fixed voltage, the channel length must be increased. Except for increasing the channel resistance, it would make the process harder.
In this thesis we invent the retrograde-body profile. It utilizes the high energy implantation to obtain that the body concentration near drain side is high and is low near source side. The calculation results of simple model exhibit that the depletion width is diminished substantially due to the high body concentration near drain side. Therefore we can shorten the channel length and it would be beneficial for device dimension scaling down. In addition, the high body concentration near drain side has almost been depleted as the device in saturated mode. The threshold voltage is determined by the body concentration at saturated-point, so we can decrease the threshold voltage, shorten the channel length and the on-resistance would be lower at the same time. After that we would obtain bigger driving-current.
We find some important phenomena from actual measurement as listed below: (1)the standard deviation of threshold voltage would be bigger with the lower trench depth, but the variation is acceptable.(2)Except for diminishing trench depth, we should decrease the thickness of drift region for lower on-resistance. But the breakdown voltage would be lower. In the design of retrograde body, if we want to obtain the breakdown more than 30V, the difference of up-layer n drift region and trench depth should be larger than 0.8m. Thus, the trade-off of on-resistance and breakdown voltage is decided by the need of device performance.(3)We find the phenomenon of drain induce barrier lowing(DIBL) as the channel length is shorter due to the shallower trench depth. When drain bias a large voltage, the leakage current of the retrograde-body device at gate biasing 0V would be larger, but the value is still in the allowable range of power MOSFET.
The device of retrograde body profile can obtain nice improvement in device scaling down of perpendicular direction, driving current, on-resistance, and the switching performance. At the specifications for the threshold voltage is 1V and the breakdown voltage is larger than 30V, the trench depth can be shortened to 0.7m, the on-resistance would be 0.36mcm2 (it is lower about 40% than standard process), and the Rds,on‧QGS of switching performance is lower about 70% than standard process. So we provide a new design for the improvement of trench gate power MOSFET. Moreover, the high energy implantation would also avoid the drive-in process of body implantation with high temperature and long time which the standard process has to do, and this would be good for the reliability of device. If combined with the optimum drift region thickness, we can decrease the on-resistance about 40% than traditional process. Furthermore, the switching performance has been improved, too.
This thesis develops a new modulation on optimum doping distribution of body of trench gate power MOSFET. It can shorten the channel length, lower the on-resistance effectively, and also decouple the design of channel length and threshold voltage. In practical application, if combining it with another method which decreasing the cell pitch to increasing cell density, we would obtain greater effect on reducing on-resistance.
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